Fantasy Zone is a shoot 'em up game that was initially developed by Sega as an arcade game and is the first game in the Fantasy Zone series. It later received ports for home systems such as the NES, Sega Master System, and PC Engine. The game is best known for its cute aesthetic that it popularized for shoot 'em ups, and for popularizing the boss rush concept, where multiple bosses are fought in succession without breaks.
In this run, The8bitbeast exploits a bug in the Sega Master System version where the demos that play when you start the game can be manipulated so that they actually beat the levels they're showing off, which somehow counts as progress for the player. Since the demos show off all the levels in the game, including the final one, a huge portion of the game can be skipped. You can read the author's notes for further details.